1. Field of the Invention
The invention relates to solid state laser rods and more particularly concerns a segmented laser rod and the method of its manufacture.
2. Brief Description of the Prior Art
Satisfying a growing demand for solid state laser (Light Amplication by Stimulated Emission of Radiation) rods currently requires relatively expensive investment in straight forward capacity expansion. This is due, in part, to the present state-of-the-art methods of manufacture. For example, the commercially-practiced method of manufacturing YAG boules from which the yag laser rods are cored is the so-called "Czochralski" method. In this method a "seed" crystal is dipped into a high temperature molten pool of the doped garnet material and very slowly rotated and withdrawn. A crystal boule of about 1 to 3 inches in diameter is formed, the length of which depends on the mass of starting melt and available process time. It takes about 2-3 weeks of steady, uninterrupted processing to produce a boule of 6-8 inches in length from which usable laser rods can be "cored" along the axial (long) direction of the boule. Any interruption or significant change in process parameters during that time is likely to introduce a flaw in the boule at the position of the boule/melt interface. This is therefore a factor which determines the maximum length and total number of rods (i.e. the "yield") which can be cored from a boule. The mathematically-inclined will readily see that if process interruptions are random events in time, the process yield decreases exponentially with desired boule (rod) length. Operating at constant yield, increases in production can be achieved only by increasing the number of growth stations.
Also, the Czochralski method generally produces a boule whose quality and concentration of active dopant changes along the boule length This is due to the difference in solubility of the active dopant species in the melt as contrasted with the solid crystal boule. This is called segregation. The degree of non-uniformity along the boule depends upon the relative mass of the final boule to the mass of the starting melt.
Because of these features of monolithic boule growth, with YAG, for example, the yield of rods of even a few inches length is generally less than 50% of the geometrically-available boule material, and the overall yield of highest quality rods suitable, for example, for use in cw-pumped, TEM .sub.oo mode laser operation is even lower. The currently-used process for producing YAG rods penalizes length by ever decreasing yield. One-inch long rods are easy, six-inch lengths very difficult and costly and ten-inch rods virtually impossible to obtain. Similar problems occur with other methods of boule growth such as the Vernuil method.
A further problem occurs with long rods, namely, a dynamic effect called "thermal focusing". The longer the rod, the more the rod focuses, and the more difficult it is to maintain the laser resonator's stability and high quality beam.
Dynamically, an optically pumped (excited) solid state laser rod, by virtue of parasitic heat generated within the rod volume, and the removal of this heat by cooling through the lateral surface, shows a radial temperature gradient. This temperature gradient through the stresses created and the photo-optical properties of the material, causes the rod to exhibit the properties of a lens. In the case of YAG, the lens is positive and causes a focusing effect. The focal length F of this dynamically-produced effect is closely represented by the expression ##EQU1## where L is the length of the rod and .alpha. is a parameter that depends on material properties and the level of excitation (pumping).
Since this dynamic lens is included within the laser resonator, indeed, a part of it, there exist values of F for which the resonator becomes unstable and laser action ceases or decreases markedly. This effect is often observed as a decrease in beam output with an increase in excitation (pumping).
The method of the present invention permits one to solve the above-described problems of the prior art. All of the above-described problems are greatly reduced or entirely eliminated by abandonment of the monolithic rod in favor of a composite rod, built up from sections or segments. When only short (.about.1") active segments are required to build a long, composite rod, the boule yield problem virtually vanishes, dopant concentration variation can be eliminated by matching of sections, compensation of dynamic focusing and birefringence can be incorporated and rods of arbitrary length can be fabricated and usefully operated.